During discharging of a battery, e.g., a lithium ion battery, total voltage at the battery pack may decrease sharply due to overvoltage at certain laminating parts at which degradation has occurred. Thereby, total voltage reaches a lower limit voltage, which is set in order to prevent over-discharging, and the discharging system stops discharging considering that the battery pack is fully charged.
However, as degradation of the battery progresses the battery pack is typically not yet fully discharged (i.e., state of charge (SOC)>0%) when discharging is stopped as a result of reaching the lower limit voltage. Therefore, available amount of energy from the battery decreases due to degradation dispersion, among others.
US 2004/0038123, also published as JP 2004-087238, is directed to a stack type battery with a plurality of unit cells stacked in a stack direction to be connected in series, and shared voltage measurement tab electrodes formed on the plurality of unit cells, respectively, to allow voltages to be measured for the plurality of unit cells such that the shared voltage measurement tab electrodes are disposed at deviated positions on a side surface of the stack type battery in a direction intersecting the stack direction. However, the system of US 2004/0038123 requires voltage sensors between each laminated portion, thereby resulting in high cost. In addition, the higher the number of laminations, the more difficult it becomes to place a voltage sensor therebetween.